The appearance of superconducting materials having high critical temperature, in particular above the temperature of liquid nitrogen, has given rise to hopes of making current limiters that are cheap.
Unfortunately, at present, it is technically possible to fabricate such materials only in the form of solid tubes.
Superconducting limiters have been described that operate inductively, comprising a magnetic circuit having on one branch thereof a winding that carries the current to be limited and whose other branch is surrounded by a coil of superconducting material carrying a direct current. Under normal conditions, the magnetic circuit is saturated by the current carried by the superconducting tube; in the event of a fault, the amplitude of the current in the winding causes the magnetic circuit to desaturate, and inductive voltages in the winding serve to limit the current.
That solution is limited to low powers since at greater powers the volume of the magnetic circuit and of the primary coil become prohibitive.
Proposals have also been made for systems similar to the above, in which the superconducting coil does not carry direct current but forms a closed loop and is equivalent to a transformer having a secondary winding that is short-circuited. It thus carries a current that is proportional to the current in the primary, which current serves to screen the magnetic circuit; when the secondary current reaches the critical current, then the superconductor switches to a resistive state, thereby eliminating the screening and producing a large current-limiting impedance in the primary. That technique does indeed serve to simplify the superconducting coil, reducing it to one or more solid tubes. Nevertheless, as in the preceding technique, it is heavily penalized by the size of the magnetic circuit and of the primary coil that are required.
Limiters are also known that operate resistively, in which the current to be limited is conveyed by a portion of superconducting material whose resistance increases suddenly when the critical density is exceeded, e.g. because of a short-circuit current.
Such limiters exist, but they make use of superconducting materials having a low critical temperature.
It has also been considered that the resistive current limiting technique could be used with a superconducting material having a high critical temperature, e.g. by using a strand of high critical temperature superconducting material having a sheath made of metal, e.g. silver; the sheath of metal is necessary to make it possible to fabricate long lengths of wire. That solution cannot be used at high powers because the metal sheath compromises the current-limiting effect.